The present invention relates to a screw vacuum pump and, more particularly, to a screw vacuum pump which is designed so that it is possible to raise the ultimate pressure.
There has heretofore been one type of screw vacuum pump which has a pair of male and female rotors rotating in mesh with each other around two parallel axes, respectively, and a casing for accommodating the two rotors, the casing having a suction port and a discharge port. This type of pump includes:
(A) screw vacuum pumps which have a process of sucking a gas from the suction port into a space defined between the rotors, and a process of compressing the gas inside the rotors; and
(B) screw vacuum pumps which have a process of transferring the sucked gas between the suction and compression processes.
All the above-described conventional screw vacuum pumps are arranged such that the suction port is closed when the space volume reaches a maximum. The type (A) of screw vacuum pump suffers from the problem that since the number of groove spaces present between the discharge and suction ports is small, the gas leaks to the suction side, and it is therefore impossible to attain a high degree of vacuum. In the type (B) of screw vacuum pump, the rotor wrap angle is increased (i.e., the rotor length is increased) to provide a transfer section inside the rotors, thereby increasing the number of groove spaces present between the discharge and suction ports. Therefore, this type of screw vacuum pump has the disadvantage that the axial length of the rotors increases, resulting in an increase in the overall size of the pump, although a high degree of vacuum can be attained.